1. Field of the Invention
The present invention relates to a valve lifter structure and, particularly to a valve lifter structure in a mechanism for varying valve characteristics, for example, timings of valve opening and valve closing, valve lift and others, using a three-dimensional cam, arranged between the three-dimensional cam and a top of a valve stem of a poppet valve.
2. Description of the Prior Art
In a known mechanism for varying valve characteristics, a three-dimensional cam having a profile varying in the axial direction is displaced in the axial direction for the purpose of changing a valve-opening/closing timing of an intake valve or an exhaust valve, formed by a poppet valve in an internal combustion engine, in accordance with operational conditions.
When such a three-dimensional cam is used, the durability of a slanted cam surface and a cam follower can be improved by providing a guide groove on a top surface of a valve lifter arranged between the three-dimensional cam and a top of the valve stem of the poppet valve so that cam and the valve lifter are brought into surface-contact with each other.
For example, Japanese Unexamined Patent Publication No. 3-179116 discloses a mechanism wherein a guide groove completely encircled by walls is formed on a top of a valve lifter and a tilting follower is arranged therein, or the guide groove has a raised portion at a center thereof. In this mechanism, a slide-contact surface between the guide groove and the follower is necessarily formed with a high degree of precision so that the follower can be smoothly tilted.
However, the former type having the guide groove completely encircled with walls has a drawback in that it is difficult to machine the slide-contact surface to a high degree of precision because the tools usable for such a purpose are limited.
On the other hand, the latter type having a raised portion has a drawback in that part of the follower is necessarily thin-walled to receive such a portion, which weakens the follower.